Flat-panel displays are widely used in conjunction with computing devices, in portable devices, and for entertainment devices such as televisions. Such displays typically employ a plurality of pixels distributed over a display substrate to display images, graphics, or text. In a color display, each pixel includes light emitters that emit light of different colors, such as red, green, and blue. For example, liquid crystal displays (LCDs) employ liquid crystals to block or transmit light from a backlight behind the liquid crystals and organic light-emitting diode displays rely on passing current through a layer of organic material that glows in response to the current. Displays are typically controlled with either a passive-matrix (PM) control employing electronic circuitry external to the display substrate or an active-matrix (AM) control employing electronic circuitry formed directly on the display substrate and associated with each light-emitting element. Both OLED and LC displays using both passive-matrix control and active-matrix control are available. An example of such an AM OLED display device is disclosed in U.S. Pat. No. 5,550,066.
Active-matrix circuitry is commonly achieved by forming thin-film transistors (TFTs) in a semiconductor layer formed over a display substrate and employing a separate TFT circuit to control each light-emitting pixel in the display. The semiconductor layer is typically amorphous silicon or poly-crystalline silicon and is distributed over the entire flat-panel display substrate. The semiconductor layer is photolithographically processed to form electronic control elements, such as transistors and capacitors. Additional layers, for example insulating dielectric layers and conductive metal layers are provided, often by evaporation or sputtering, and photolithographically patterned to form electrical interconnections, or wires.
Typically, each display sub-pixel is controlled by one control element, and each control element includes at least one transistor. For example, in a simple active-matrix organic light-emitting diode (OLED) display, each control element includes two transistors (a select transistor and a power transistor) and one capacitor for storing a charge specifying the luminance of the sub-pixel. Each OLED element employs an independent control electrode connected to the power transistor and a common electrode. In contrast, an LCD typically uses a single transistor to control each pixel. Control of the light-emitting elements is usually provided through a data signal line, a select signal line, a power connection and a ground connection. Active-matrix elements are not necessarily limited to displays and can be distributed over a substrate and employed in other applications requiring spatially distributed control.
The amount of light emitted from an LCD is determined by the brightness of the backlight, the transmissivity of the liquid crystals, and the area of the display through which light is emitted. A larger pixel area will transmit more light than a smaller pixel area. Hence, in order to achieve a desirably bright LCD, the pixel areas are preferably large. In contrast, the brightness of an OLED display depends on the current density passed through the OLED pixels. At higher current densities, brightness is increased and lifetime is decreased. Thus, a larger light-emitting OLED area will increase the lifetime of an OLED display by reducing the current density or enable an increased current and brightness without increasing the current density or reducing the OLED lifetime. It is therefore also preferred that OLED pixels are large.
The percentage of a display area that is given over to the actual light-controlling pixel area is known as the aperture ratio or fill factor. A larger aperture ratio typically results in a longer lifetime or a greater maximum brightness for a flat-panel display. Low aperture ratios are especially problematic for displays in which the light-controlling elements (e.g. light-transmissive or light-emitting elements) are located on the same substrate as the active control elements (such as thin-film transistors), for example a bottom-emitting OLED display in which the display substrate area is shared between the control circuits and the light-emitting area. Active-matrix LC displays suffer from the same problem and, in addition, the active control elements are located between the light emitter (the backlight) and a viewer. In an active-matrix LC display the TFTs are located between a viewer and the backlight and occupy space on the control substrate. In both cases, an increased brightness is required for a given display brightness when the aperture ratio of the display is relatively smaller. The increased brightness results in a reduced lifetime. Thus, in both cases, a larger aperture ratio is preferred.
Inorganic light-emitting diode displays using micro-LEDs (for example having an area less than 1 mm square, less than 100 microns square, or less than 50 microns square or having an area small enough that it is not visible to an unaided observer of the display at a designed viewing distance) are also known. For example, U.S. Pat. No. 8,722,458 entitled Optical Systems Fabricated by Printing-Based Assembly teaches transferring light-emitting, light-sensing, or light-collecting semiconductor elements from a wafer substrate to a destination substrate.
Displays often incorporate or are used in combination with other technologies that enhance the usefulness of the combined system. In particular, many portable display systems include touch screens. Users view information on the display and then indicate an associated action by physically touching a screen located over the display where the information is displayed. The touch is detected by an external controller and the appropriate action corresponding to the displayed information is taken.
Although displays with associated functionality are widespread in the computing and telecommunications industries, there remains a need for display systems, structures, and methods of manufacturing that provide increased lifetime, image quality, ease of manufacturing, reduced cost and improved functionality in simpler and more robust structures.